Map_Kd not passed correclty in mtl file
Blender allows us to pack image and other data files into the blend file.

An image texture that is packed into a blend file and not saved to disk, will export to .obj with map_Kd . in the .mtl file.

The solution is to view the image in blenders UV/Image editor and save it out to a file. Image->Save As Image or press F3.

You can also use File->External Data->Unpack All Into Files to unpack multiple files in one go.